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By amplifying small quantities of Aβ oligomer in cerebrospinal fluid, a methodology referred to as the "bio-barcode" could serve as a diagnostic or monitoring test for Alzheimer disease, according to a study published this week in the PNAS Early Edition. In the paper, a team of chemists at Northwestern University in Evanston, Illinois, led by Chad Mirkin and collaborating with Bill Klein's group at the same institution and researchers at Rush University Medical Center in Chicago, finds preliminary evidence that levels of Aβ oligomers (or amyloid-β-diffusible ligands, ADDLs) reliably distinguished AD patients from control subjects in an initial sample, with little of the overlap that has hobbled prior efforts to find biochemical AD biomarkers.

The bio-barcode method developed by Mirkin and colleagues (Nam et al., 2003) is designed to amplify extremely low levels of protein or oligonucleotide in solution. The method involves getting the scarce proteins to bind to gold particles festooned with polyclonal antibodies—in this case, ADDL antibodies developed by Klein and colleagues, who have found ADDLs to be elevated in AD brain and in AD transgenic mice (Chang et al., 2003; Gong et al., 2003; also see ARF related news story). The gold particle has also been tagged with thousands of identical single strands of DNA, which amplify the ADDL signal. A specific sequence has been chosen to uniquely identify ADDLs, thus the barcode analogy. Mirkin and colleagues suggest that many different scarce proteins can be assayed at once, each having a unique DNA "barcode."

The ADDL-gold-DNA cluster is isolated by means of another molecule complex—a magnetic particle tagged with ADDL monoclonal antibodies. Magnets pull the combined complex out of solution, and the DNA is separated from the gold and quantified. Mirkin's group reports that the bio-barcode method has proven capable of detecting attomolar quantities. That equals some 18 to 20 copies of a given protein in 10 microliters of solution, according to the authors.

Klein and colleagues hypothesized that traces of ADDLs might diffuse out from brain tissue into the CSF. In the current pilot study, led by first author Dimitra Georganopoulou, the researchers found that ADDL concentrations in the CSF of 15 AD patients were consistently higher than those in 15 age-matched controls. Only two of the AD patients had an ADDL concentration in the range of the control people: One of these had little clinical evidence of the disease (evidenced by a high MMSE score), and the other had pathological evidence of infarctions.

The authors note that any other scarce pathogenic AD markers found in the CSF could also be assayed in this fashion, potentially providing more powerful predictive value. Also, they write, "it suggests that the soluble pools of ADDLs that exist in the human brain extend to the CSF, and that elevated levels of ADDLs correlate with the presence of the disease."—Hakon Heimer

Comments on News and Primary Papers

This paper reports the fruits of a wonderful collaboration between an Alzheimer researcher (Bill Klein) and a "nanochemist", Chad Mirkin, both at Northwestern University. It follows very important work by Bill Klein and his colleagues implicating amyloid beta diffusable ligands (ADDLs)in the pathogenesis of the synaptic loss associated with Alzheimer's disease. Using the extraordinarily sensitive "Bio-Bar Code-based DNA Detection" system (J-M Nam et al., J Am Chem Soc 126:5932, 2004) with a pair of monoclonal and polyclonal antibodies to ADDLs, they were able to differentiate a group of 15 AD CSF specimens from 15 controls, with the exception of two possibly anomalous AD cases. The authors caution readers that a much larger study will be required. It would be especially important to include subjects with Mild Cognitive Impairment. The sensitivity of the method is such that there is reason to believe that useful diagnosic discriminations might be made using specimens of peripheral blood.